102
A. Patel, R. Sadasivan / Inorganica Chimica Acta 458 (2017) 101–108
K7[PW10Ti2O40] was found to be difficult. Synthesis of Na2{-
CoHbhep(H2O)4}[PW10Co2O38{Hbhep}2] and Na2{NiHbhep(H2O)4}
[PW10Ni2O38{Hbhep}2] has been reported by Cronin et al. [21].
The typical synthesis employs acidification of solution of ligand,
individual salts of tungstate and cobalt with nitric acid followed
by addition of phosphoric acid. After a long time of reflux (18 h),
red crystalline material has been obtained. Recently, Patel et al.
established one-pot synthesis of Cs7[PW10Mn2(H2O)2O38]ꢀ7H2O
by mixing commercially available H3PW12O40ꢀ29H2O and MnCl2-
ꢀ4H2O at pH 6.4, followed by reflux and addition of CsCl [22].
A literature survey shows that since last 22 years no study has
been carried out on the di-copper substituted phosphotungstates
even though copper contains distinct advantages such as unique
redox properties, relatively inexpensive and low toxic nature.
Our group has been working on substituted polyoxometalates
since last five years. We have synthesized and characterized a
number of mono transition metal substituted polyoxometalates
and studied their catalytic activities for a number of organic trans-
formations. As an extension, it was thought to be of interest to syn-
thesize di-copper substituted phosphotungstate by one pot
synthesis method developed by our group [22] and to evaluate
its catalytic activity. We have also attempted to develop a new
and sustainable synthesis strategy using the microwave route.
A literature survey also shows that no catalytic study has
been carried out for any of the di-substituted phosphotungstates.
So, it was thought to be of interest to evaluate the catalytic
activity of the said compound. Epoxidation of cis-cyclooctene
was selected for the same. The reason for the selection are (i)
epoxides are important intermediates for the manufacture of
many bioactive molecules and fine and bulk chemicals for
polymer synthesis [23] and (ii) our expertise for carrying out
oxidation reactions.
The present paper reports synthesis of Cs salt of di-copper sub-
stituted phosphotungstate by the reaction of H3PW12O40 and CuCl2
in H2O, followed by the addition of CsCl by two procedures: (1)
direct one pot synthesis and (2) microwave synthesis. Both the
products have been analyzed by various spectroscopic techniques
such as FT-IR, 31P NMR, ESR and CV. We have also compared the
FT-IR, ESR, 31P NMR and CV data of the products obtained by both
the synthetic techniques and have found that the products
obtained in both the methods are the same. However, we have
been unable to get good quality crystals so far for single crystal
analysis. Work is still in progress for the same. A detailed study
was carried out to explore the use of synthesized complex for
the oxidation of cis-cyclooctene with tert-butyl hydroperoxide
(TBHP) as the oxidant under mild reaction conditions.
was refluxed for 2 h at 100 °C (Scheme S1). This was filtered hot,
then 0.5 g solid CsCl was immediately added. The resulting green-
ish blue precipitates were filtered, dried at room temperature and
designated as RPW10Cu2 (Yield-44.6%).
2.3. Microwave synthesis of di-copper substituted phosphotungstate
2.88 g (1 mmol) of H3PW12O40ꢀnH2O was dissolved in 10 mL of
water and the pH of the solution was adjusted to 6.4 using super-
saturated NaHCO3 solution. The solution was heated to 90 °C with
stirring. To this hot solution, 0.32 g (2 mmol) of CuCl2ꢀ2H2O dis-
solved in minimum amount of water, was added. The solution
was microwaved at 100 °C for 30 s (Onida PC23 Black Beauty,
power output – 800 W) with 5 intervals of 10, 5, 5, 5 and 5 s. This
was filtered hot, then 0.5 g solid CsCl was immediately added
(Scheme S2). The resulting greenish blue precipitates were filtered,
dried at room temperature and designated as MPW10Cu2 (Yield-
43.8%).
In order to verify the viability of the technique, the same com-
plex was synthesized using copper acetate instead of copper chlo-
ride. Cu(CH3COO)2ꢀH2O (0.39 g; 2 mmol) was added in place of
CuCl2ꢀ2H2O and the material was synthesized using the same pro-
cedure as above. Greenish powder was obtained, which was desig-
nated as MPW10Cu2Ac.
2.4. Catalyst characterization
The materials synthesized by both the reflux and microwave
techniques were characterized by Elemental analysis, Thermal
analysis (TG-DTA), Fourier transform Infrared spectroscopy, 31P
NMR Spectroscopy, UV–Visible spectroscopy, Cyclic Voltammetry,
Powder X-ray Diffractometry and Electron Paramagnetic Reso-
nance Spectroscopy.
Elemental analysis was carried out using a Perkin Elmer
Optima-3300 RL ICP Spectrometer (for Cu and W) and JSM 5610
LV EDX-SEM analyzer (for Cs). TG-DTA was carried out on the Met-
tler Toledo Star SW 7.01 up to 600 °C. FT-IR spectra of the sample
were obtained by using the KBr pellet on the Perkin Elmer instru-
ment. 31P solution NMR was carried out in D2O solvent on a Bruker
AVANCE 161.97-MHz instrument. The UV–Vis spectrum was
recorded at room temperature on Perkin Elmer 35 LAMDA instru-
ment using the 1 cm quartz cell in range of 200–1100 nm. The
powder XRD pattern was obtained by using PHILIPS PW-1830.
The conditions were: Cu Ka radiation (1.54 Å), scanning angle from
0° to 60°. ESR spectra were recorded on a Varian E-line Century ser-
ies X-band ESR spectrometer (Liquid nitrogen temperature and
scanned from 2000 to 3000 Gauss). Cyclic voltammetry was per-
formed on CHI660E Electrochemical Workstation, Model 600E Ser-
ies. A glassy carbon working electrode, an Ag/AgCl reference
electrode Pand Pt-wire counter electrode were used.
2. Experimental section
2.1. Materials
All chemicals used were of A.R. grade. H3PW12O40ꢀ29H2O,
CuCl2ꢀ2H2O, CsCl and dichloromethane were obtained from Merck.
cis-cyclooctene was obtained from Spectrochem Pvt. Ltd., Mumbai.
NaHCO3 was obtained from SRL, Mumbai. All chemicals were used
as received.
2.5. Catalytic reaction
The oxidation reaction of cis-cyclooctene was carried out in a
50 mL batch reactor with a double walled air condenser, magnetic
stirrer and a guard tube. TBHP was used as the oxidant. 15 mg of
the precatalyst was added. Owing to the fact that there was no
aqueous medium, the precatalyst remained heterogeneous. The
products obtained were extracted with dichloromethane by
repeated extractions. The obtained products were analyzed on a
gas chromatograph (Shimatzu-2014), having flame ionization
detector, using a capillary column (RTX-5) and were identified by
comparison with the authentic samples.
2.2. One pot synthesis of di-copper substituted phosphotungstate
2.88 g (1 mmol) of H3PW12O40ꢀnH2O was dissolved in 10 mL of
water and the pH of the solution was adjusted to 6.4 using super-
saturated NaHCO3 solution. The solution was heated to 100 °C with
stirring. To this hot solution, 0.32 g (2 mmol) of CuCl2ꢀ2H2O dis-
solved in minimum amount of water, was added. The solution